1. Field of the Invention
The present invention relates to an image reading apparatus which reads an image by radiating the light onto an original and, more particularly, to improvements in an arrangement of a light source to illuminate the original and in a driving apparatus of the light source.
2. Description of the Prior Art
Recently, in image reading apparatuses such as optical input means for a facsimile apparatus, a copying machine or the like, demand for miniaturization has been increasing. To satisfy such demand, in many cases, an LED array in which a plurality of LED chips are arranged is used for illumination of the original surface in place of a fluorescent lamp, which has been used conventionally.
FIG. 1 shows an example of such a conventional image reading apparatus. In the diagram, a reference numeral 1 denotes an LED array serving as an illumination light source; 2 is a lens array of the refractive index distribution type serving as an optical system; 3 is a photo sensor array; 4 is an original; and 5 is a roller for conveying the original. The original 4 is illuminated by two LED arrays 1 which are arranged upstream and downstream in the original conveying direction of the lens array 2. The light reflected by the original surface is formed as an image on the photo sensor array 3 by the lens array 2 and an image on the original is read. Hitherto, in such an image reading apparatus using the LED arrays for illumination of the original surface as mentioned above, two LED arrays have had to be used to derive a sufficient quantity of light to read. However, the use of two LED arrays obviously results in an increase of the cost of the whole apparatus.
On the other hand, if only a single LED array is provided and a great amount of current is allowed to flow throught the LED chip to produce a sufficient amount of light, the temperature of the LED array will be increased and this will contrarily cause a decrease in the light amount output due to deterioration of the LED chip.
For example, in Japanese Patent Unexamined Publications Nos. 114665/1983 to 114667/1983, to eliminate such a drawback, there has been proposed an apparatus in which an LED array is divided into a plurality of blocks and these blocks are allowed to sequentially flicker in correspondence to the reading scan of the photo sensor array. By driving the LED array in this way, a sufficient light quantity to read an image can be obtained using only a single LED array, and at the same time an increase in temperature of the LED array is averted and a deterioration of the LED can be prevented.
However, in such a conventional apparatus, the LED array is driven in consideration only of the case where the light response leading time .tau..sub.on of the photo sensor is so short that it can be ignored as compared with the time S of a single reading scan of the photo sensor array. Therefore, this apparatus cannot be applied to the case where a photo sensor in which the foregoing leading time .tau..sub.on is not negligible as compared with the scan time S (for instance, .tau..sub.on .gtoreq.S/10) is used or where a photo sensor of the signal accumulation type is used. Particularly, even in the case of the non-signal-accumulation type photo sensor, the ratio of the leading time .tau..sub.on to the scan time S becomes high in association with high-speed image reading; thus, a problem is caused in the conventional LED array driving process.